Why is there weightlessness in space?

The idea that gravity stops acting beyond the atmosphere is very persistent. In reality, Earth's pull extends without limit; it only decreases with the square of the distance. At just over 400 kilometers of altitude, where the International Space Station orbits, it's barely weaker than at the surface, and it's precisely that pull that keeps the station in its orbit.

An orbit is nothing more than a very, very long throw. If a stone is thrown horizontally, it's seen falling to the ground describing an arc. If it goes fast enough, the Earth's surface curves downward beneath the trajectory just as fast as the stone falls, so it never actually touches the ground. Newton described this with a cannon placed on a very high mountain, long before rockets existed.

The crew falls exactly like their spacecraft. Nothing pushes them against the seat, no floor supports them, and the inner ear no longer detects any direction. The same thing happens in an elevator whose cable breaks, except there the free fall ends within seconds.

Weightless doesn't mean massless. A toolbox has the same inertia on the station as on Earth: it can be pushed with a pinky finger, but stopping it again takes the same effort. That's why astronauts always work tethered, clipped in, or held with velcro tape.

Free fall is never perfect. Residual atmosphere, orbital maneuvers, fans and crew movements generate tiny accelerations that can still disturb sensitive experiments. That's why specialists prefer to speak of microgravity rather than weightlessness. A laboratory falling without propulsion around the Earth is, nonetheless, much closer to the ideal condition than any facility on the ground.